Determining actual GPM is a bit of an itterative process because to get accurate information, you have to estimate what your working pressure at the spray head is going to be, then use charts for the spray head to determine GPM. Add up all the GPMs running through all the pipes, determine pressure losses in the pipes. Use the pressure losses to determine working pressure. Use working pressure to estimate GPM at each spray head. Use GPM of each spray head to determine GPM through each pipe, use GPM through each pipe to determine pressure losses in each pipe, use pressure losses to estimate working pressure at each spray head...